Conventionally, many types of furnishings and interior materials are decorated by coating the surface of their substrates, such as wood boards, with an adhesive and then applying a laminate such as a film, decorative paper or laminating material thereon.
Roller coaters are generally used for applying adhesive to a substrate such as a wood board. Roller coaters have two rollers for adjusting the application quantity of the adhesive, namely, an applicator roller and a metering roller, and a backing roller that applies pressure to the substrate and also conveys the substrate.
A water emulsion is generally employed as the adhesive used in this regard. The water emulsion is applied to a substrate such as a wood board to form an adhesive layer, with a film or decorative paper and the like is applied thereon. The resulting laminated object possesses a degree of smoothness and aesthetic appeal making it suitable for use in various furnishings and interior materials. In recent years, the demands on aesthetic appearance have increased, with a higher degree of surface smoothness being sought. As a result, conventional water emulsions used as adhesives are no longer up to the task.
Plywood, medium density fiber board (MDF), particle board, and the like, are used as the wood board to which the aforementioned film or decorative paper is bonded. Among these, the surface of plywood is a veneer having minute surface indentations. Applying an adhesive such as a water emulsion directly to the surface of plywood with an applicator roller results in minute indentations in the adhesive layer, inhibiting the formation of a smooth adhesive layer of uniform thickness. Such minute indentations, by showing through the surface of the applied film or decorative paper, are deleterious to smoothness and aesthetic appeal.
In order to remedy this defect, so-called sealing processing is performed involving the application of a sealer to the surface of a wood board to fill in concavities. This to some extent improves the smoothness of the surface, but still falls short of the exacting demands for smoothness in recent years as long as adhesives such as conventional water emulsions are used.
In addition, applying water emulsion adhesives to medium density fiber board (MDF) causes swelling of the medium density fiber board (MDF), which adversely affects smoothness.
Water emulsions generally contain certain amounts of organic solvent such as toluene or xylene. Their use in interior furnishing materials requires drying time after application of a film or decorative paper. In addition, their use in interior furnishing materials causes sick house syndrome. Furthermore, drying equipment is costly and providing installation space for such equipment is often difficult.
Solvent adhesives also have problems such as giving off a bad smell when dried and the residual solvent of the volatile organic compound being carcinogenic.
Besides the above water emulsion adhesives and solvent adhesives, hot melt adhesives such as ethylene vinyl acetate, polyamide, reactive urethane and reactive epoxy adhesives that solve problems related to sick house syndrome and volatile organic compounds are also available.
However, their higher viscosity compared to other adhesives leads to the problem of roughening occurring easily on the surface of the substrate. That is, when the applicator roller comes in contact with the surface of the wood board and when it is retracted from the board after applying the adhesive, the adhesive itself and the surface of the substrate are easily roughened due to the high viscosity of the adhesive. As a result, surface indentations after bonding of the film or decorative paper become noticeable, adversely affecting the aesthetic appeal. This problem becomes prominent when using hot melt adhesives.
Water emulsion adhesives are self leveling due to their high application quantity and relatively low viscosity. In contrast, hot melt adhesives are highly viscous, have a short open time after being applied to the surface of a wood board, and exhibit a sudden drop in temperature after application. Due to the accompanying sudden rise in viscosity, self leveling cannot be expected. Obtaining a smooth and thin adhesive layer of uniform thickness is therefore difficult, with roughening remaining as surface irregularities after the film or decorative paper is applied, adversely affecting the aesthetic appeal as an interior furnishing material.
It is conceivable to apply the adhesive to the back of the film or decorative paper, instead of the surface of the wood board, as a way to solve these problems. However, compared to applying the adhesive to the surface of the wood board, permeation of the adhesive applied on the film side into the wood board is insufficient, resulting in diminished bonding strength.
As described above, it is difficult to form a smooth and thin adhesive layer of uniform thickness on a substrate such as a wood board using conventional adhesives and coating equipment, and there remains scope for improving the smoothness of surfaces to which a film or decorative paper has been subsequently bonded.
Also, use of water emulsion adhesives or solvent adhesives entails a drying process, leading to problems related to drying equipment and providing space for such equipment as well as prolonging the work process. Problems such as sick house syndrome also arise from use of organic solvents.